Vehicle driven by synchronous motors

ABSTRACT

The wheels of a vehicle are driven by inductor type synchronous motors having solid rotors without windings and commutators. A diesel on the vehicle drives a pair of high frequency rotating generators each of which supplies power to a pair of the synchronous motors. Power pedal means actuated by the operator derive a power signal proportional to the desired tractive effort for the vehicle. A separate electric drive for each motor includes means for deriving a control signal modulated at motor speed, a cycloconverter between each motor and its generator regulated by the control signal, tachometer means for deriving a speed signal which is a function of motor speed, and control means responsive to both the power signal and speed signal for regulating the magnitude and phase of the control signal so that the motor output power is constant over the speed range at a level in accordance with the power pedal setting. Brake means actuated by the operator derive a brake signal, and the vehicle has means for shifting the phase of the control signals for all the motors to regeneratively brake them when the brake signal exceeds the power signal. The vehicle has travel direction selector means for shifting the phase of all the control signals to reverse the direction of motor rotation and thus propel the vehicle backward. The vehicle has manually operated means for setting a speed limit for the vehicle and means for deriving a speed limit signal which is additive to the brake signal to regeneratively brake the motors when vehicle speed exceeds the set speed limit. Turn compensation means increase the power supplied to the motors of the wheels on the outside of a turn and decrease the power supplied to the motors of the wheels on the inside of the turn as a function of the degree of turn and the magnitude of the power signal.

United States Patent William L. Ringland Greendale;

Manfred E. Neumann, New Berlin; Ernst K. Kaeser, West Allis; Thomas P. Gilmore, Wauwatosa; Allois F. Geiersbach,

[72] inventors [54] VEHICLE DRIVEN BY SYNCHRONOUS MOTORS 58 Claims, 34 Drawing Figs.

[52] U.S.Cl 318/146,

318/148,3l8/l68,3l8/l7l,318/231 [51] Int.Cl H02p 5/22 [50] FieldofSearch 318/146- Primary Examiner-Gris L. Rader Assistant ExaminerK. L. Crosson Attorneys-Lee H. Kaiser, Robert B. Benson and Thomas F.

Kirby ABSTRACT: The wheels of a vehicle are driven by inductor type synchronous motors having solid rotors without windings and commutators. A diesel on the vehicle drives a pair of high frequency rotating generators each of which supplies power to a pair of the synchronous motors. Power pedal means actuated by the operator derive a power signal proportional to the desired tractive effort for the vehicle. A separate electric drive for each motor includes means for deriving a control signal modulated at motor speed, a cycloconverter between each motor and its generator regulated by the control signal, tachometer means for deriving a speed signal which is a function of motor speed, and control means responsive to both the power signal and speed signal for regulating the magnitude and phase of the control signal so that the motor output power is constant over the speed range at a level in accordance with the power pedal setting.

Brake means actuated by the operator derive a brake signal;

and the vehicle has means for shifting the phase of the control [56] References Cited signals for all the motors to regeneratively brake them when UNITED STATES PATENTS the bralte signal exceeds the power signal. The vehicle has 3 164 760 1/1965 King 318/231 travel dniecnon Selector means sinftmg the phase f the 3402333 9/1968 Havner et a] 318/171 control signals to reverse the direction of motor rotation and 3443184 5/1969 Lelnmrich 318/23] thus propel the vehicle backward. The vehicle has manually 237O078 2/l945 Schaelchlin' 318/231 operated means for setting a speed limit for the vehicle and 205O6 5/1967 Humphrey 318/231 means for deriving a speed limit signal which is additive to the 3394297 7/1968 Risberg 318/231 brake signal to regeneratively brake the motors when vehicle 3477002 1 H1969 318/231 speed exceeds the set speed limit. Turn compensation means 3484669 liq/i969 Roberts 318/176 increase the power supplied to the motors ofthe wheels on the outside of a turn and decrease the power supplied to the mo- FOREIGN PATENTS tors of the wheels on the inside of the turn as a function of the 745,840 3/1956 Great Britain 318/231 degree of turn and the magnitude ofthe power signal.

Bid/r: Paw;

PHI! monw I I l 1 l l l I I I I l l PATENTED HAY SHEET PATENTEU MAY 4 I971 SHEET [13 0F 14 PATENTEDIAY 4m! 3,577,050

sum 07 0F 14 MNMN a wm PATENIEU HAY 4:971

saw as or 14 PATENTEU HAY 4l97| SHEET 10 OF 14 Mum VEHICLE DRIVEN BY SYNCHRONOUS MOTORS This invention relates to electrically driven vehicles and in particular to vehicles driven by synchronous electric motors.

This application is a continuation-in-part of our application Ser. No. 824,223, entitled Vehicle Electric Motor Drive System," filed May 13, 1969 and having the same assignee as this application.

Electrically driven vehicles may employ a diesel, gasoline, or turbine engine as the primary source of power, or prime mover. The prime mover may drive one or more generators which, in turn, power one or more electric motors connected to the wheels or tracks of the vehicle. On wheeled vehicles it is often advantageous to utilize a separate motor for each wheel. An electric control may be interposed between the enginedriven generators and the motors of such a vehicle to regulate the application of electrical power to the motors and wheels.

In the past, most electrically driven vehicles have utilized direct current electrical apparatus. Direct current motors, particularly those of the series types, are readily adaptable to electric drive systems because of their case of control, and further the electrical controls for direct current motors and generators are simple and well developed. However, the commutators and rotating armature coils required by direct current machines add to their manufacturing and maintenance cost.

Electrically driven vehicles are known which employ alternating current motors that are simple and reliable in construction. However, since the speed of an alternating current motor is determined by the frequency of the-altemating current source, a control must be provided to convert the fixed frequency of the source to the required motor frequency which varies with speed.

The simplest type of alternating current motor is the induction motor, but the differential, or slip, between the speed of the rotor and that of the rotating stator field presents difficulties in using a frequency detector driven by the motor shaft for controlling the frequency converter which supplies the variable frequency to the motor stator. The stator frequency must be higher than the detected rotor frequency by the amount of the slip frequency, and further the slip results in heat losses in the rotor which are difficult to remove, particularly at low speeds. The torque of an induction motor is proportional to the square of the voltage-to-frequency ratio applied to the stator winding. For constant power and constant r.p.m. slip conditions, the voltage applied to an induction motor over a desired speed range must increase as the square root of the applied frequency. In a typical instance of 16 to 1 frequency (and motor speed) range at constant power, the maximum supplied voltage required is four times the minimum voltage. lnasmuch as the size of the power supply in a drive system, including the generator and power controlling elements, is determined by the maximum voltage required as well as the maximum current required, a large power supply is necessary for an electric drive system using induction motors.

Because the speed of a synchronous motor is inherently proportional to applied frequency, synchronous motors have generally been employed in constant speed drive systems where the motor is energized from a constant frequency source such as a 60 cycle per second power line. When energized at this frequency, the torque of a synchronous motor as su'mes a level sufficient to cause the motor to drive the load in synchronism with the rotating stator field. Such conventional synchronous motor operation is one of constant speed, variable power due to the variable motor torque.

It is an object of the invention to provide an electrically driven vehicle which has the following advantages over vehie. ease of maintenance and repair;

1. adaptability to different types of prime movers and vehicles;

- g. location of powered wheels and the prime mover not restricted by the mechanical transmission elements, thereby permitting use in articulated and tandem vehicles;

h. power applied to the tractive elements of the vehicle may be individually selected and controlled, thereby permitting steering by controlling the power and speed of the tractive elements on either side of the vehicle;

i. electric braking may be provided to the vehicle thereby reducing the wear, maintenance, and required capability of the mechanical brakes.

It is a further object of the invention to provide a vehicle driven by electric motors which have no armature coils, commutator rings or brushes and are capable of rotation at higher peripheral speeds and are easier to cool than other types of electric motors heretofore used to propel vehicles.

It is another object of the invention to provide a vehicle driven by electric motors of the synchronous type which are easily controlled to produce torque directly proportional to the voltage-to-frequency ratio applied to the stator winding.

It is a further object of the invention to provide a vehicle driven by electric motors of the synchronous type which are readily controlled to provide constant power over a wide speed range without increasing the applied voltage, thereby permitting use of a small power supply.

it is another object of the invention to provide an electric vehicle driven by synchronous motors wherein motor speed is always directly proportional to the frequency of the applied voltage and thus frequency detecting means driven by the motor can be efficiently utilized to control the frequency converter which supplies variable frequency power to the motor.

It is still another object to provide a vehicle driven by alternating current electric motors of the synchronous type which provide better power factor and efficiency than induction motors.

A still further object of the invention is to provide a vehicle having a variable frequency electric drive system which utilizes a frequency changer that permits bidirectional power flow to provide regenerative braking.

A further object is to provide a vehicle having an electric drive system that transmits the maximum available power of the prime mover to the tractive elements at all vehicle speeds.

Another object is to provide such a vehicle having an electric drive system wherein, under constant power from the prime mover, maximum torque is provided to the vehicle under starting or low speed conditions.

Another object of the invention is to provide a vehicle having a variable frequency electric propulsion system utilizing a synchronous motor which permits use of a generator and a frequency changer having a voltage rating substantially lower than that of known electric drive systems of equal horsepower.

A further object of the invention is to provide an electrically driven vehicle having speed limit means which permit the operator to select the maximum speed at which the vehicle will travel and to do so even though the power pedal is fully depressed, thereby assuring the safety of the operator when the vehicle is traveling down grade.

It is a further object of the invention to provide an electrically driven vehicle wherein the operator may select the direction of vehicle travel and set a maximum speed limit for the vehicle by manipulating a single control lever.

A still further object of the invention is to provide an electrically driven vehicle having means for regeneratively braking the electric motors controlled by either a brake pedal or by manually set speed limit means.

Another object of the invention is to provide an electrically driven vehicle having means to increase the torque of outside wheels and to decrease the torque. of the inside wheels on a turn as a function of the angle through which the front wheels are turned and the vehicle power being developed, thereby facilitating turning of the vehicle.

Still another object of the invention is to provide an electrically driven vehicle wherein the torque of the electric motors which propel the wheels is controlled by a manually operated power pedal.

A further object is to provide such a vehicle wherein the power pedal actuates means for selectively deriving a power signal which controls the torque of the motors, and wherein manually controlled brake pedal means selectively derive a brake signal which cancels the power signal at a predetermined partial brake pedal position even though the power pedal is fully depressed.

. Another object is to provide such a vehicle having means for controlling the power output of the electrical motors as a function of the difference between the power signal and the brake signal.

Still another object is to provide such a vehicle having means for comparing the brake and power signals to derive a difference signal and brake sense means responsive to one polarity of said difference signal to regeneratively brake the electric motors.

Another object of the invention is to provide an electrically driven vehicle in which the phase of the voltage applied to the electric motors is shifted 180 to regeneratively brake the motors and thus brake the vehicle.

A still further object of the invention is to provide an electrically driven vehicle in which the phase of the voltage applied to the electric motors is shifted to reverse the direction of the motors and thus drive the vehicle in reverse.

Another object is to provide such an electrically driven vehicle wherein the amount of power flowing in reverse through the frequency converter to regeneratively brake the motors is a function of the brake pedal signal regardless of whether the vehicle is being driven forward or in reverse.

A further object is to provide an electrically driven vehicle having a prime mover, electrical generator means driven by the prime mover, an electric motor operatively connected to each tractive element, and control means between the generator means and each motor for independently regulating the power supplied to the motor.

Another object is to provide such an electrically driven vehicle wherein each motor assumes a speed in accordance with the load on the wheel driven by it even though each control means receives the same input control signal.

Another object is to provide such a vehicle having means associated with each motor for generating an output signal which is a function of motor speed, a frequency converter between the generator means and the motor controlled by the output signal, and control means for selectively varying a condition of the output signal.

Still another object is to provide such a vehicle having means for selectively deriving a power signal which is a function of the desired tractive effort of the vehicle, means associated with each motor for generating a speed signal which is a function of its speed, and wherein said control means for each motor independently regulates the phase and magnitude of the output signal which controls the frequency converter as a function of the power signal and the speed signal.

These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram in block form of the electric drive system of a vehicle embodying the invention;

FIG. 2 is a sectional view through a vehicle wheel and the electric motor which drives it in the embodiment of FIG. 1;

FIG. 3 is an exploded perspective view of the electric motor and the angle sensor driven by it in the embodiment of FIGS. 1 and 2, the motor stator and windings being shown as encapsulated in resin;

FIG. 4 is a partial front view showing the rotor and stator of the angle sensor;

FIG. 5 is a graph plotting relative torque provided by the electric drive system versus relative speed;

FIG. 6 is a graph showing the no-load saturation curve and the rated armature current, zero power factor saturation curve for the electric motor of the embodiment of FIGS. 1-5;

' FIG. 7a shows the simplified equivalent circuit of the electric motor; FIG. 7b shows its vector diagram for motor operation; and, FIG. 7c shows its vector diagram for generator operation;

FIG. 8 is a graph plotting motor terminal voltage and displacement angle versus speed required to provide maximum rated power over the speed range;

FIG. 9 is a development of the angle sensor stator and rotor and also schematically illustrates the instantaneous voltages generated in the angle sensor secondary windings;

FIGS. 10a and 10b are schematic diagrams illustrating vector addition of the two input signals to the angle sensor primary windings to derive the output signal which controls the cycloconverter;

FIG. 11 is a graph plotting the motor and angle sensor voltages versus motor speed required to obtain constant 50 percent and percent maximum power over the speed range;

FIG. 12 is a schematic diagram in block form of the angle sensor control, the discriminator, and the clipping circuit;

FIG. 13 is a schematic circuit diagram of the function generators of the angle sensor control;

FIG. M is a schematic circuit diagram of the cycloconverter and filter and showing the firing circuit in block form;

FIGS. 15a through 15h schematically illustrate voltages in the cycloconverter of the electric drive system;

FIG. 16 is a schematic circuit diagram of the tachometer;

FIG. 17a is a schematic circuit diagram, partially in block form, of the firing circuit for the controlled rectifiers of the cycloconverter; and FIGS. 1711 through 17j show signals at various points within the firing circuit of FIG.

FIG. 18 is a simplified schematic diagram of the relay logic circuit and the motor control;

FIG. 19 is a graph plotting the output voltage VR from current control amplifier versus motor speed;

FIG. 20 is a graph plotting motor field current versus power pedal position to provide constant power over the speed range for a selected power level;

FIG. 21 shows curves plotting as polar coordinates the variation of motor terminal voltage VT and displacement angle DT with motor speed illustrated in FIG. 8;

FIG. 22 is a schematic circuit diagram of the turn compensation means; and

FIG. 23 is a schematic circuit diagram of the speed limit circurt.

GENERAL DESCRIPTION Referring to FIG. 1, a vehicle embodying the invention has an electric drive system 10 which provides motive power to the right front, left front, right rear and left rear wheels 12, 14, 16 and 18 of the vehicle, respectively. A prime mover on the vehicle is preferably a hydrocarbon-fueled engine such as a gas turbine or a diesel engine 20 which operates at substan tially constant speed, as determined by the setting of its governor, and is capable of providing constant power input to electric drive system 10 under all conditions of vehicle operation.

Electric drive system 10 preferably includes four electric controls, or drives 22, 24, 26 and 28 (shown in block form) for the wheels l2, 14, 16 and 18 respectively. All four drives are substantially identical, and only drive 22 for right front wheel 12 will be described.

Prime mover 20 drives a first generator 30 which provides electrical power for electric drives 22 and 24 that operate front wheels 12 and 14 and also drives a second generator 32 which provides electrical power for electric drives 26 and 28 which operate the vehicle rear wheels 16 and 18. Generators 30 and 32 are similar and may be conventional high frequency polyphase alternators, and only generator 30 will be described. Generator 30 preferably has a stationary exciting coil 34 energized from a suitable power supply (not shown) from conventional driver-operated vehicle controlling ap paratus including a steering wheel 40, a travel direction and speed limit selector 42, a power pedal 44, and a brake pedal 46 which are coupled through a master driver-operated control circuit 48 to electric drives 22, 24, 26 and 28.

Electric drive 22 includes a synchronous motor 50 which preferably is of the inductor type and has a rotor52 mechanically coupled to right front wheel l2 through appropriate gearing (shown in FIG. 2), a three-phase armature winding, or stator winding 54, and a field winding 56 mounted on the motor stator. Electric drive 22 converts the constant frequency, constant voltage output of high frequency generator 30 to a variable frequency, variable voltage, variable phase altemating current for application to armature winding 54 to regulate the torque and speed of synchronous motor 50.

Electric drive system 10 transmits the constant power output of prime mover 20 to the vehicle wheels 12, l4, l6 and 18 over a wide range of speed. The hyperbolic torque versus speed characteristic of electric drive system 10 to accomplish such transmission of constant power from prime mover 20 to the vehicle wheels over a speed range of IS to l is shown in FIG. 5. Torque and speed are conveniently related for purpose of description to base values taken at the minimum speed at which constant maximum power is transmitted. The base rating of electric drive system 10 wherein relative torque is arbitrarily designated 1.0 and relative speed designated 1.0 is shown in FIG. 5 wherein the full line curve corresponds to the maximum power capability of diesel engine 20 with power pedal 44 fully depressed and with total power equally divided between the individual wheel drives 22, 24, 26 and 28. How ever, drive system 10 includes means described hereinafter to reduce motor torque under operating conditions requiring less than rated power as represented by the dotted line curve designated reduced power" in FIG. 5. Vehicle speed, motor speed, and motor frequency are all directly proportional because of the use of synchronous motors 50 and the fixed ratio gearing between motors 50 and wheels I2, 14, 16 and 18. Further, motor torque and vehicle tractive effort are also directly proportional because of such fixed ratio gearing.

In the preferred embodiment of the invention motor 50 is of the synchronous inductor yp although any type synchronous motor having adjustable field excitation, including the conventional salient pole type, may be used in electric drive system 10. The conventional saturation curves of synchronous motors are commonly used to show the relationship between stator terminal voltage VT and field current I; for various load conditions. Since synchronous motors are usually operated at constant speed corresponding to a terminal voltage having a fixed frequency of 60 cycles per second, such saturation curves are usually shown for a fixed frequency. The effect of armature resistance on these synchronous motor characteristics is usually small but becomes increasingly significant as the frequency approaches zero.

When synchronous motor 50 is operated at variable frequency, as in electric drive system 10, more meaningful saturation curves which illustrate the effects of frequency variation and armature resistance at low speeds can be derived by replacing the terminal voltage VT by the variable (VT+I,,R )/F as illustrated in FIG. 6 which shows the no-load saturation curve and the rated armature current, zero power factor saturation curve for a typical synchronous motor 50 suitable for electric drive system 10 and where:

' VT is the motor terminal voltage;

I, is the armature current;

R is the effective armature resistance; and

F is the frequency.

It will be noted that the values shown in FIG. 6 are expressed as per unit quantities relative to the base rating of the motor.

The vector quantity V'I"+Y,,R, is an internal voltage, commonly called the voltage behind the resistance, and represents the vector sum of the terminal voltage and the armature resistance drop, observing the appropriate phase angle between the two vectors. Commonly used convention considers the power component of armature current to be positive for generator operation and negative for motor operation. Therefore, for motor operation the vector VT-l-LR, is less than VT for power factors in the normal operating range near unity where VT and L, are displaced approximately (see FIG. 7b). The total quantity (VI+T,,R )/F is proportional to the net magnetic flux linking the armature winding of motor 50, which net magnetic flux is the resultant of that generated by the armature winding 54 and field winding 56.

The saturation curves of FIG. 6 permit close approximation, by well-known methods, of the field current I, corresponding to any load condition. In the preferred embodiment of the invention, electric drive 22 holds the field current I, in motor 50 constant for a desired armature current corresponding to a given position of power pedal 44. As motor speed and frequency increase above the base value F=l.0, the electric drive 22 of the preferred embodiment also holds the terminal voltage VT constant for a given power pedal position, thereby resulting in the net magnetic flux, as represented by variable (VT-HUM decreasing very nearly inversely proportional to frequency F. These conditions are represented in FIG. 6 for rated armature current by the vertical line at I,=2.5 designated constant I, for I,,=l.0, and it will be noted that the variable (WERHF decreases from a value of 1.0 at frequency F=l .0 along this constant field current line to a value of 0.5 at frequency F= 2.0 and then decrease further to a value of 0.2 at F=5. Below frequency F=l.0, the terminal voltage VT is reduced by electric drive 22 as described hereinafter but at a rate that results in the net flux rising to a maximum as the frequency approaches zero. With the field current I, and terminal voltage VT constant, the armature current I, is close to rated value (I,,=l .0) and the power factor close to unity over the entire range of frequency, thereby resulting in the electric drive system 10 having the hyperbolic constant power speedtorque characteristic shown in FIG. 5. Assuming that electric drive system 10 is designed to have a power rating matching that of diesel engine 20, this speed-torque curve of FIG. 5 might alternatively be designated maximum electric drive system continuous capability." When less than rated power is required to drive the load and power pedal 44 is not fully depressed, the speed-torque characteristic may be represented by the dotted line curve designated reduced power" in FIG. 5.

Electric drive 22 regulates the terminal voltage VT applied to armature winding 54 of electric motor 50 as described above and also shifts the applied terminal voltage VT through the required phase angle relative to the angular position of the rotor to obtain the constant power torque-speed characteristic shown in FIG. 5. Such required phase angle is best described by reference to FIG. 7a, which shows the simplified equivalent circuit of synchronous motor 50 under steady state conditions, and to FIGS. 7b and 70 which show its vector diagram for motor and generator operation respectively and wherein:

E is the internal voltage proportional to the field current;

X, is the effective synchronous reactance for all field positions relative to the armature m.m.f.; and

R,, is the effective armature resistance.

The armature current 1,, results from the vector voltage difference actin on the machine impedance, and thus:

FIG. 7!: illustrates the vector diagram of this relationship for motor operation approximating the conditions in electric drive 22 at rated load for speed F=l.0, and FIG. 7c illustrates the vector diagram for generator operation approximating the conditions in electric drive 22 at speed F=l .0, with braking power less than rated load. The displacement angle DT is the phase angle between the internal voltage E and the the terminal voltage VT applied to stator winding 54.

Electric drive 22 advances the tenninal voltage VT in phase by the angle (+)DT relative to the physical axis of the internal voltage E,, on the rotor in order to produce the conditions for motor operation illustrated in FIG. 7b, and it delays the terminal voltage in phase by the angle (-)DT relative to the physical axis of E in order to provide the conditions for generator action shown in FIG. 70.

In the equivalent circuit and vector diagram of FIG. 7, the internal voltage E and effective synchronous reactance X,

are proportional to frequency and are multiplied by F for operation at other than base frequency F=l .0.

FIG. 8 illustrates the terminal voltageVT and displacement angle DT which electric drive 22 applies to stator winding 54 of synchronous motor 50 at rated load to provide the desired constant power speed-torque characteristic of FIG. with constant field current, essentially constant annature current and power factor close to unity. It will be noted that the displacement angle DT is 0 at F=0.0 and approaches 90 at high frequency and motor speed. FIG. 8 shows only two VT curves designated rated power" and reduced power, but it will be appreciated that a different VT curve exists for each position of power pedal 44 and the corresponding power output level from motor 50.

Electric drive 22 includes a frequency changer, or cycloconverter 58 which is supplied with constant voltage, high frequency power from generator 30 over buses 38 and is responsive to gating signals from a firing circuit 60 to convert this high and constant frequency power to a lower variable frequency terminal voltage VT supplied over conductors 62 to stator phase windings 54X, 54Y and 54Z of synchronous motor 50. Cycloconverter 58 is shown in detail in FIG. 14 and preferably includes a positive group of three thyristors, or silicon controlled rectifiers associated with each of the three motor stator phase windings 54X, 54\ and 542 to carry positive current from the three-phase power busses 38A, 38B and 38C and a negative group of three silicon controlled rectifiers associated with each of these three motor stator phase windings to carry negative current from the busses 38A, 38B and 38C.

Firing circuit 60 derives gating signals which cyclically fire the silicon controlled rectifiers in cycloconverter 58 at desired points in the cycles of the high frequency, constant magnitude voltages A, B and C in busses 38A, 38B and 38C to generate the three-phase voltages in conductors L1, L2 and L3 which are applied to stator phase windings 54X, 54V and 54Z.

Electric drive 22 includes a rotary inductor, vector adder, or resolver, termed angle sensor" 64 driven by motor 50 for deriving a control signal for cycloconverter 58 which is a replica in magnitude, frequency and phase of the voltage VT (shown in FIG. 8) to be applied to stator winding 54 to obtain constant power over the speed range. In order to keep the poles generated in motor rotor 52 locked in with the revolving poles generated by motor stator winding 54, the frequency of the terminal voltage VT applied to stator winding 54 must at all times be in synchronism with rotor speed, and further the terminal voltage must be advanced in phase at all motor speeds by the displacement angle DT between the magnetic flux produced by the field current acting alone and the magnetic flux corresponding to the terminal voltage. Further, the magnitude of the terminal voltage VT impressed on stator winding 54 must be controlled as a function of motor speed in the manner shown in FIG. 8.

The variation in magnitude VT and phase angle DT of the terminal voltage shown in FIG. 8 to be applied to stator winding 54 can be expressed graphically by the loci of an equation in which motor speed is the variable parameter and the magnitude VT and phase angle DT are the radius vector and vectorial polar coordinates of the curve formed by the loci. FIG. 21 shows such a curve in dotted lines designated rated power" plotting the loci of terminal voltage VT and phase angle DT when motor 50 is delivering rated. power and also shows a curve in full lines designated reduced power.- The magnitude of the terminal voltage VT to'be applied to stator winding 54 as a function of motor speed is the radius vector of such curve, two such vectors VT1 and VT2 for the reduced power curve being shown in FIG. 21. The displacement angle by which the terminal voltage is to be advanced in phase relative to the rotor poles as a function of motor speed is the vectorial angle of the curve, two vectorial angles DTl and DT2 for the reduced power curve being shown. It will be noted that the magnitude of the terminal voltage VT is maintained constant from the base speed F=I .0 at the lower limit of the speed range (shown by vector VTI having phase angle DTl) with increase in motor speed, while the displacement angle increases from approximately 40 at F=l.0 to approximately at speed F=3.5, at which the terminal voltage is shown by the radius vector VT2 and the displacement angle by the vectorial angle DT2.

A plurality of different curves can be plotted in FIG. 21 all of which are of the same shape and each of which represents a different position of power pedal 44 and a corresponding different power output level from motor 50. Each curve shown in FIG. 21 can also be defined by its rectangular coordinates x and y which vary as a function of motor speed F, or by the parametric equations of the curve having motor speed F as the variable parameter. Electric drive 22 includes an angle sensor control 76 which generates a pair of sine and cosine" signals V, and V, representative of the y and x rectangular coordinates of a curve of FIG. 21 for each position of power pedal 44, and angle sensor 64 vectorially adds such signals and derives an output signal for controlling cycloconverter 58 whose magnitude and phase are in accordance with the radius vector and vectorial angle polar coordinates of such curve. Inasmuch as a curve of FIG. 21 is the loci of an equation which expresses the desired variation in magnitude VT and phase angle DT with motor speed as shown in FIG. 8, the terminal voltage VT applied by cycloconverter 58 to motor stator winding 54 is in accordance with one of the curves of FIG. 8 corresponding to a given power pedal position. As explained in detail hereinafter, the output signals from angle sensor 64 regulate firing circuit 60 which derives the gating signals for firing the thyristors of cycloconverter 58.

Rotary inductor, vector adder, or angle sensor 64 in effect converts the signals V, and V representative of the y and x rectangular coordinates of a curve of FIG. 21 into the polar coordinates of such curve. Angle sensor 64 has a secondary winding 66 comprising three Wye-connected secondary phase windings 66X, 66Y and 662 displaced (electrical) and a pair of energizing, or primary windings, termed sine winding 68 and cosine winding 70, displaced 90 electrically from each other magnetically coupled with secondary winding 66. The primary windings 68 and 70 and the three phase secondary windings 66X, 66Y and 662 are wound on all angle sensor stator 72 (see FIGS. 2, 3 and 4) mounted on the housing of motor 50, and the magnetic flux linkage between the secondary winding 66 and the primary windings 68 and 70 depends on the airgap between the stator 72 and a ferromagnetic rotor 74 connected to the motor rotor 52. The angle sensor rotor 74 is contoured to produce an approximately sinusoidal variation in the air gap and in the flux linkage between each secondary phase winding 66X, 66Y and 66Z and the primary windings 68 and 70 as it rotates.

The sine and cosine primary windings 68 and 70 are separately excited with high frequency, inphase sine and cosine signals V, and V from angle sensor control 76 which are in accordance with the y and x rectangular coordinates respectively of a curve of FIG. 21 corresponding to a given position of power pedal 44. If motor 50 is at standstill and the angle sensor control 76 were to energize the sine and cosine windings with fixed magnitude, in-phase, high frequency alternating signals, the 90 displaced sine and cosine windings 68 and 70 would have constant ampere turns and induce fixed magnitude, high frequency signals in the three-phase secondary windings 66X, 66Y and 662. The permeances of the paths for the magnetic flux generated by the sine and cosine windings 68 and 70 and the voltage level of the fixed amplitude signals induced in the three-phase secondary windings is a function of the position of angle sensor rotor 74. When motor 50 rotates, the high frequency angle sensor output voltages induced in secondary windings 66X, 66Y and 662 are no longer fixed in amplitude but rather have a sinusoidal modulation at a relatively low frequency which is representative of the speed of the motor 50. The envelopes of the three angle sensor output signals induced in the secondary phase windings 66X, 66Y and 662 are displaced l (electrical) because of the physical location of these windings on the angle sensor stator 72.

The term output voltage" is used herein to connote either the carrier frequency signals induced in secondary phase windings 66X, 66Y and 66Z, their low frequency sinusoidal modulation envelopes, or the signals derived therefrom by demodulation to remove the carrier frequency and alternate half waves. The carrier frequency signals induced in secondary phase windings 66X,-66Y and 662 are designated vr, and their low frequency modulation envelopes and the signals derived therefrom by demodulation are designated VT, Further, all three such forms of output voltage" are directly proportional in magnitude, and their magnitude is represented in curves by the designation VT,

Angle sensor control 76 receives a reference "power" signal from protection and regulation circuit 78 (see FIG. 1) which is a function of the position of power pedal 44 and independently controls the signals V, and V, to sine and cosine windings 68 and 70 of angle sensor 64 as a function of this power signal, thereby controlling the ampere turns of these windings and the magnitude of the signals vt, (and their modulation envelopes VT,) induced in secondary phase windings 66X, 66Y and 662 which, after demodulation, control cycloconverter 58. Thus angle sensor control 76 suitably regulates the magnitude of the signals V, and V,. to the sine and cosine windings 68 and 70 of the angle sensor 64 to control the magnitude of terminal voltage VT applied to the motor stator winding 54 as a function of power pedal position. Change in position of power pedal 44 varies the magnitude of the power signal and thus changes the length of the radius vector VT in FIG. 21.

Angle sensor control 76 also receives a speed" signal from tachometer 80 (see FIG. 1) which is a function of the speed of the motor 50 and modifies the signals V and V to the sine and cosine windings 68 and 70 in response to the speed signal in accordance with c y and x rectangular coordinates of a curve of FIG. 21 so that the angle sensor output voltages VT, induced in secondary windings 66X, 66Y and 662 vary linearly (see FIGS. 8 and 11) from zero at zero speed to full value (corresponding to a given position of power pedal 44) at base frequency F=l.0 and remain at full value from F=l .0 to maximum motor speed F=l5.0. Above base speed F=l.0, angle sensor control 76 unequally varies the magnitude of the signals V, and V to the sine and cosine windings 68 and 70 in opposite directions as a function of motor speed (see FIG. 11) while maintaining the magnitude of the angle sensor output voltage VT, constant for a given power pedal position, thereby unequally varying the ampere turns and the intensity of the magnetic flux which these windings 68 and 70 generate and shifting the phase of the modulation envelopes VT, of the angle sensor output voltages induced in secondary windings 66X, 66Y and 662 relative to the angular position of the angle sensor rotor 74 while maintaining the magnitude of these voltages constant. The output signals VT, from angle sensor secondary winding 66 control cycloconverter 58, and angle sensor control 76 thus regulates the displacement angle DT of the voltage VT applied to motor stator winding 54 relative to tits muons-tic mla: m mntnr rotor 52 as a function of motor speed, while maintaining the magnitude of the terminal voltage-VT constant for a given position of power pedal 44, by independently varying the magnitude of the sine and cosine signals V, and V to sine and cosine windings 68 and 70 to obtain the terminal voltage versus speed and displacement angle versus speed characteristics designated VT and DT in FIG. 8.

The three-phase carrier frequency output voltages v1, from angle sensor secondary windings 66X, 66Y and 662 are demodulated in a discriminator 82 to remove the carrier signal. Discriminator 82 provides three-phase output voltages VT, of motor frequency controlled in magnitude and phase angle relative to a reference axis or the motor rotor, i.e., the axis of internal motor voltage E shown in the vector diagram of FIG. 7b. a

The peak voltages of the low frequency output signals VT, from discriminator 82 are limited in a clipping circuit 84 which derives flat-topped output signals VT, (see FIG. 12) when the voltages from the discriminator 82 exceed the clipping level.

When the speed of motor 50 can be regulated over a speed range wherein the motor frequency voltages VT, applied to stator winding 54 vary from zero to f Hertz, the generator 30 provides voltages A, B and C having a frequency of at least 2f and preferably 3f when diesel 20 is controlled by its governor to run at top speed.

In order to condition the generator high frequency, constant magnitude voltages A, B and C in the busses38A, 38B and 38C for generating the synchronizing, or sequence" signals required by firing circuit 60, a filter 86 (see FIG. 1) removes the commutation notches and high frequency noise from the generator voltages A, B and C and regulates the voltage level thereof to derive reference" voltages which are reproductions of the fundamental waves of these generator voltages. Firing circuit 60 combines these three-phase, high frequency output reference signals from filter 86 with the three-phase low frequency control signals VT, from clipping circuit 84 (which are proportional to the angle sensor output voltages VT,,) to derive the sequence" signals A+VT,, B+VT, and C+VT, shown in FIG. I5e, and the firing circuit 60 includes level detectors (described hereinafter) which sense the zero crossing points of such sequence signals and generate the gating signals for the controlled rectifiers of the cycloconverter 58.

As explained hereinbefore, the terminal voltages VT applied to stator winding 54 and displacement angles DT shown in FIG. 8 will result in the desired speed-torque characteristic of FIG. 5 with constant field current, essentially constant armature current, and power factor close to unity. Voltage drops occur in generator 30 because of the commutating inductance, and additional resistance and reactance voltage drops occur in the controlled rectifiers and center tapped reactors of cycloconverter 58, and the effect of such voltage drops on motor terminal voltage VT and on the controlling input signal voltage to cycloconverter 58 is approximately the same as that of an added resistance in series with the motor. The output voltage VT from cycloconverter 58 to motor stator winding 54 is proportional to, and a replica of, the angle sensor output voltage which controls cycloconverter 58 and would have the magnitude versus speed response of the VT curve in FIG. 8 if such voltage drops and the limiting of the peak voltages in clipping circuit 84 were not taken into consideration. In order to compensate for such voltage drops under load and also for the clipping of the VT, signals in clipping circuit 84, angle sensor control 76 increases the magnitude of the output signal from angle sensor 64 for rated load from a signal having the magnitude of the full line VT curve to that shown by the curve designated VT, and also varies the phase angle of the angle sensor output signal (which controls cycloconverter 58) in accordance with the curve designated DT, in FIG. 8 rather than along a characteristic similar in shape to the DT curve.

The position of power pedal 44, brake pedal 46 and travel direction selector 42 determine the magnitude of a DC 

1. In combination, a land vehicle having a ground-engaging tractive element, at least one inductor type synchronous motor on said vehicle having a polyphase stator winding, a rotor operatively connected to said tractive element, and means for generating magnetic poles in said rotor, a prime mover on said vehicle, a polyphase alternator on said vehicle driven by said prime mover, means operatively connected to said rotor for generating an output signal having a frequency which is a function of the speed of said motor, a polyphase frequency converter between said alternator and said stator winding controlled by said output signal, and control means for selectively varying a condition of said output signal.
 2. In the combination of claim 1 wherein said rotor is solid and ferromagnetic and said means for generating magnetic poles in said rotor includes a field winding on the stator of said motor.
 3. In the combination of claim 2 wherein said output signal generating means derives a polyphase output signal, and said frequency converter is a cycloconverter having controlled rectifiers between each phase of said stator winding and said alternator.
 4. In the combination of claim 3 wherein said control means includes means for deriving reference signals which are functions of the phase voltages generated by said alternator, means for combining said reference signals and the phase output signals to provide composite sequence signals, and means controlled by said sequence signals for deriving gating signals for said controlled rectifiers.
 5. In the combination of claim 4 wherein said gating signal deriving means includes means for detecting when said sequence signals cross the zero axis and wherein the maximum frequency of said output signal over the speed range of said motor is F cycles per second, and including governor means for controlling the speed of said prime mover so that said alternator generates a polyphase voltage having a frequency of at least 2F cycles per second.
 6. In the combination of claim 5 and including means for selectively deriving a power signal which is a function of the desired tractive effort of said vehicle and whereIn said control means is responsive to said power signal and varies a condition of said polyphase output signal as a function of said power signal.
 7. In the combination of claim 6 and including tachometer means for deriving a speed signal which is a function of the speed of said motor and wherein said control means regulates the magnitude and shifts the phase of said output signal in accordance with said power signal and said speed signal.
 8. In the combination of claim 7 and including manually operated means for selectively driving a brake signal, and wherein said control means includes a subtractive amplifier for comparing said power and brake signals and generating a difference signal which is a function of their difference, and wherein said control means regulates the magnitude and shifts the phase of said polyphase output signal in accordance with said difference signal and said speed signal and regulates the energization of said field winding in accordance with said difference signal.
 9. In the combination of claim 8 wherein said subtractive amplifier provides a difference signal of one polarity when said brake signal is greater than said power signal and of the opposite polarity when said power signal is greater than said brake signal and wherein said control means includes absolute value amplifier means responsive to either polarity of said difference signal for providing an absolute value signal whose magnitude is a function of the magnitude of said difference signal, and wherein said control means regulates the magnitude and shifts the phase of said polyphase output signal as a function of said absolute value signal and said speed signal and also controls the energization of said field winding as a function of said absolute value signal.
 10. In the combination of claim 9 wherein said control means includes brake sense means responsive to said one polarity of said difference signal for shifting the phase of said polyphase output signal 180* to effect regenerative braking of said motor.
 11. In the combination of claim 10 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings and in which said output signal is induced and said brake sense means includes means for reversing both of said energizing windings to shift the phase of said output signal 180*.
 12. In the combination of claim 9 and including manually operated travel direction selector means for shifting the phase of said polyphase output signal to effect reversal of the direction of rotation of said motor and the direction of travel of said vehicle.
 13. In the combination of claim 12 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings and in which said output signal is induced and said travel direction selector means includes means to reverse one of said energizing windings to shift the phase of said output signal.
 14. In the combination of claim 9 and including protective means responsive to an undesirable operating condition in said vehicle for deriving a fault signal, and wherein said control means includes summing amplifier means receiving said absolute value signal and said fault signal as inputs and providing a summing signal which is a function of their sum, and wherein said control means regulates the magnitude and shifts the phase of said polyphase output signal as a function of said summing signal and said speed signal and controls the energization of said field winding as a function of said summing signal.
 15. In the combination of claim 8 and including means for selectively deriving a speed limit signal which is a function of the desired maximum speed of said vehicle, speed limit means for comparing said speed signal and said speed limit signal and for deriving a speed limit error signal when the former exceeds the latter, said sPeed limit error signal being an input to said subtractive amplifier and being additive to said brake signal input.
 16. In the combination defined by claim 2 and including means for selectively deriving a power signal which is a function of the desired tractive effort of said vehicle and wherein said control means is responsive to said power signal.
 17. In the combination of claim 16 and including tachometer means for deriving a speed signal which is a function of the speed of said motor and wherein said control means regulates the magnitude and shifts the phase of said output signal in accordance with said power signal and said speed signal.
 18. In the combination of claim 16 and including manually controlled means for selectively deriving a brake signal, and wherein said control means includes a subtractive amplifier for comparing said power and said brake signals and for generating a difference signal which is a function of their difference, and wherein said control means varies a condition of said output signal and regulates the energization of said field winding as a function of said difference signal.
 19. In the combination of claim 18 wherein said subtractive amplifier provides a difference signal of one polarity when said brake signal is greater than said power signal and of the opposite polarity when said power signal is greater than said brake signal and wherein said control means includes absolute value amplifier means responsive to either polarity of said difference signal for providing an absolute value signal whose magnitude is a function of the magnitude of said difference signal, said control means varying a condition of said output signal as a function of said absolute value signal and also regulating the voltage applied to said field winding as a function of said absolute value signal.
 20. In the combination of claim 19 wherein said control means includes brake sense means responsive to said one polarity of said difference signal for regeneratively braking said synchronous motor.
 21. In the combination of claim 18 wherein said control means includes brake sense means responsive to said one polarity of said difference signal for shifting the phase of said output signal 180* to effect regenerative braking of said motor.
 22. In the combination of claim 21 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings and in which said output signal is induced and said brake sense means includes means for reversing both of said energizing windings to shift the phase of said output signal 180*.
 23. In the combination of claim 3 and including manually operable travel direction selector means for shifting the phase of the voltage supplied by said cycloconverter to said stator winding to reverse the direction of rotation of said motor and thereby reverse the direction of travel of said vehicle.
 24. In the combination of claim 23 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings and in which said output signal is induced and said travel direction selector means includes means for reversing one of said energizing windings to shift the phase of said output signal.
 25. In the combination of claim 19 and including protective means responsive to an undesirable operating condition in said vehicle for deriving a fault signal and wherein said control means includes a summing amplifier means receiving said absolute value and fault signals as inputs and providing a summing signal which is a function of their sum and said control means varies a condition of said output signal and controls the energization of said field winding as a function of said summing signal.
 26. In the combination of claim 18 and including manually operated means for deriving a speed limit signal whose magnitude is proportional to the Desired maximum speed of said vehicle, tachometer means for generating a speed signal which is a function of the speed of said motor, speed limit means for comparing said speed signal and said speed limit signal and for deriving a speed limit error signal when the former exceeds the latter, said speed limit error signal being an input to said subtractive amplifier and being additive with said brake signal input.
 27. In combination, a land vehicle having an earth-engaging tractive element, at least one synchronous electric motor on said vehicle having a stator winding and a rotor operatively connected to said tractive element, a prime mover on said vehicle, electric generator means including an alternator on said vehicle driven by said prime mover, means on said vehicle for selectively deriving a power signal which is a function of the desired tractive effort of said vehicle, manually controlled means for selectively deriving a brake signal, subtractive amplifier means for comparing said power and brake signals and for generating a difference signal which is a function of their difference, and control means between said electric generator means and said stator winding for regulating the voltage delivered by said generator means to said stator winding as a function of said difference signal.
 28. In the combination of claim 27 wherein said subtractive amplifier means generates a difference signal of one polarity when said brake signal is greater than said power signal and of the opposite polarity when said power signal is greater than said brake signal, and wherein said control means includes absolute value amplifier means responsive to either polarity of said difference signal for providing an absolute value signal whose magnitude is a function of the magnitude of said difference signal, said control means regulating the voltage delivered by said generator means to said stator winding as a function of said absolute value signal.
 29. In the combination of claim 27 wherein said means for deriving a brake signal is responsive to the magnitude of said power signal and, when it is at a predetermined partially operated position, derives a magnitude of brake signal greater than the magnitude of said power signal when said power signal deriving means is fully operated.
 30. In the combination of claim 29 wherein said means for deriving a power signal includes a potentiometer, a direct current power source connected across the winding of said potentiometer, and a power signal amplifier coupled to the movable contact of said potentiometer and said means for deriving a brake signal includes a potentiometer having one end coupled to the output of said power signal amplifier.
 31. In the combination of claim 28 wherein said control means includes brake sense means responsive to said one polarity of said difference signal for regeneratively braking said electric motor.
 32. In the combination of claim 31 and including a cycloconverter between said generator means and said stator winding of said motor regulated by said control means and said brake sense means is responsive to said one polarity of said difference signal to shift the phase of the voltage supplied by said cycloconverter to said stator winding 180* to effect regenerative braking of said motor.
 33. In the combination of claim 32 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings for deriving a control signal for said cycloconverter and said brake sense means includes means for reversing both of said energizing windings to shift the phase of the voltage supplied by said cycloconverter 180*.
 34. In the combination of claim 32 and including manually operable travel direction selector means for shifting the phase of the voltage supplied by said cycloconverter to said stator winding to reverse the direction of rotation of said Motor and thereby reverse the direction of travel of said vehicle.
 35. In the combination of claim 34 wherein said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings and in which said output signal is induced and said travel direction selector means includes means for reversing one of said energizing windings to shift the phase of said voltage supplied by said cycloconverter to said stator winding.
 36. In the combination of claim 27 and including manually operable speed limit means for selectively deriving a speed limit signal, and wherein said control means includes tachometer means for deriving a speed signal which is a function of the speed of said motor and means for comparing said speed limit signal and said speed signal and for deriving a speed limit error signal when the latter exceeds the former, said speed limit error signal being an input to said subtractive amplifier means and being additive to said brake signal input.
 37. In the combination of claim 36 and including a cycloconverter between said generator means and said stator winding of said motor regulated by said control means and travel direction selector means for shifting the phase of the voltage supplied by said cycloconverter to said stator winding to reverse the direction of rotation of said motor and thereby reverse the direction of travel of said vehicle and wherein said vehicle has a manually operable travel direction selector and speed limiting movable element, the direction of movement of said element controlling said travel direction selector means and the degree of movement thereof controlling said speed limit signal deriving means and the magnitude of said speed limit signal.
 38. In the combination of claim 27 wherein said control means includes means responsive to an undesirable operating condition in said vehicle for reducing the voltage delivered by said generator means to said stator winding below that voltage level which is a function of said power signal.
 39. In the combination of claim 28 wherein said control means includes a summing amplifier receiving said absolute value signal as an input and means responsive to an undesirable operating condition in said vehicle for deriving a fault signal which is an input to said summing amplifier, and wherein said summing amplifier provides a summing signal which is a function of the sum of said absolute value and fault signals and said control means regulates the power delivered by said generator means to said stator winding as a function of said summing signal.
 40. In the combination of claim 39 wherein said means responsive to an undesirable operating condition comprises means for detecting a predetermined magnitude of overcurrent delivered by said generator means to said stator winding.
 41. In the combination of claim 39 wherein said means responsive to an undesirable operating condition comprises means for detecting a predetermined overtemperature of said motor.
 42. In combination, a land vehicle having a plurality of ground-engaging tractive elements, a plurality of inductor type synchronous motors on said vehicle each having a polyphase stator winding, a solid ferromagnetic rotor operatively connected to one of said tractive elements, and a field winding on said motor stator for generating magnetic poles in said rotor, a prime mover on said vehicle, polyphase electric generator means on said vehicle driven by said prime mover, a plurality of means each of which is associated with one of said motors operatively connected to said rotor for deriving an output signal having a frequency which is a function of the speed of said one motor, a plurality of polyphase frequency converters each of which is connected between said electric generator means and the stator winding of one of said motors controlled by said output signal which is a function of the speed of said one motor, a plurality of Control means each of which is associated with one of said motors for varying a condition of said output signal, and manually operated means for selectively deriving a power signal which is a function of the desired tractive effort of said vehicle, each of said control means being responsive to said power signal.
 43. In the combination defined by claim 42 wherein said electric generator means includes a polyphase alternator on said vehicle connected to said motors for each pair of tractive elements, said alternator being driven by said prime mover.
 44. In the combination of claim 42 including tachometer means associated with each of said motors for generating a speed signal which is a function of the speed of said motor, and wherein each of said control means regulates the voltage and shifts the phase of said output signal as a function of said speed signal and said power signal.
 45. In the combination of claim 42 and including manually controlled means for selectively deriving a brake signal, and wherein each of said control means includes a subtractive amplifier for comparing said power and brake signals and generating a difference signal which is a function of their difference, each control means varying a condition of said polyphase output signal and controlling the energization of said field winding as a function of said difference signal.
 46. In the combination of claim 45 wherein said subtractive amplifier provides a difference signal of one polarity when said brake signal is greater than said power signal and of the opposite polarity when said power signal is greater than said brake signal and each said control means also includes absolute value amplifier means responsive to either polarity of said difference signal for providing an absolute value signal whose magnitude is a function of the magnitude of said difference signal, each said control means regulating a condition of said output signal and controlling the energization of said field winding as a function of said absolute value signal.
 47. In the combination of claim 46 and including a plurality of tachometer means each of which is associated with one of said motors for deriving a speed signal which is a function of the speed of said motor and wherein each said control means regulates the magnitude and phase of said polyphase output signal as functions of said absolute value signal and said speed signal.
 48. In the combination of claim 46 and including a plurality of cycloconverters each of which is between said generator means and the stator winding of one of said motors and is controlled by said output signal and wherein each said control means includes brake sense means responsive to said one polarity of said difference signal for shifting the phase of said output signal 180* to effect regenerative braking of said motor.
 49. In the combination of claim 48 wherein each of said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings in which said output signal is induced and said brake sense means includes means for reversing both of said energizing windings.
 50. In the combination of claim 45 wherein said vehicle has manually operated travel direction selector means for shifting the phase of all of said output signals to effect reversal of the direction of rotation of said plurality of motors and thus reverse the direction of travel of said vehicle.
 51. In the combination of claim 50 wherein each of said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings in which said output signal is induced and said travel direction selector means includes means in each of said control means for reversing one of said energizing windings to shift the phase of said output signal.
 52. In the combination of claim 45 and including manually operable means for selectivelY deriving a speed limit signal which is a function of the desired maximum speed of said vehicle and wherein each of said control means includes means for deriving a speed signal which is a function of the speed of the associated motor and also includes speed limit means for comparing said speed signal and said speed limit signal and for deriving a speed limit error signal when the former exceeds the latter, said speed limit error signal being an input to said subtractive amplifier and being additive to said brake signal.
 53. In the combination of claim 48 wherein said vehicle has a plurality of protective means each of which is associated with one of said motors and is responsive to an undesirable operating condition thereof for generating a fault signal, and each of said control means includes a summing amplifier receiving the corresponding fault signal and absolute value signal as inputs and providing a summing signal which is a function of their sum, and wherein each said control means also includes tachometer means for generating a speed signal which is a function of the speed of said motor and said control means regulates the phase and magnitude of said polyphase output signal as a function of said speed and summing signals and also includes means for applying a voltage to said field winding which is a function of said summing signal.
 54. In the combination of claim 52 wherein said manually operable means for deriving a speed limit signal also actuates travel direction selector means for shifting the phase of all of said polyphase output signals to effect reversal of the direction of rotation of said plurality of motors.
 55. In the combination of claim 54 wherein each said control means includes vector adder inductor means having a pair of energizing windings and a secondary winding inductively linked to said energizing windings in which said output signal is induced and said travel direction selector means includes means for reversing one of said energizing windings in each said control means.
 56. In the combination of claim 53 wherein each frequency converter is a cycloconverter having controlled rectifiers between each phase of said generator means and said stator winding and each said control means includes means for deriving reference signals which are functions of the phase voltages produced by said generator means, means for combining said reference voltages and the associated phase output signals to derive composite sequence signals, and means controlled by said sequence signals for deriving gating signals for said controlled rectifiers.
 57. In the combination of claim 56 wherein said gating signal deriving means includes means for detecting when said gating signals cross the zero axis, and wherein the maximum frequency of said output signals over the speed range of said motors is f cycles per second, and including governor means for controlling the speed of said prime mover so that said generator means produces a polyphase voltage having a frequency of at least 2f cycles per second.
 58. In the combination of claim 37 wherein said subtractive amplifier means generates a difference signal of one polarity when said brake signal is greater than said power signal and of the opposite polarity when said power signal is greater than said brake signal and wherein said control means includes brake sense means responsive to said one polarity of said difference signal for regeneratively braking said synchronous electric motor. 